PHYSIOLOGICAL SENSITIVITY OF PLANTS ALONG AN ELEVATIONAL GRADIENT TO UV-B RADIATION

Seeds from four plant pairs collected from contrasting elevations in Hawaii were grown in greenhouses at the University of Maryland at UV-B radiation levels that approximated a 20% and 40% stratospheric ozone depletion anticipated at sea level in Maui. In general, increases in UV-B radiation resulted in earlier reproductive effort, increased dark respiration and maintenance of relative water content (RWC), photosynthesis, and apparent quantum efficiency (AQE) in plants from higher elevations where natural UV-B radiation is already high. In contrast, plants collected from low elevational ranges showed a significant decline in average plant and floral dry biomass, a decline in AQE and RWC, and a reduction in light-saturated photosynthetic capacity. Increases in UV-B-absorbing compounds (e.g., flavonoids), were noted for low elevation but not high elevation plants. However, plants from high elevations produced a consistently larger amount of these compounds even in the absence of UV-B radiation. This study suggests that plants growing in a naturally high UV-B environment may have developed or maintained mechanisms related to reproductive phenology and carbon uptake which may maintain productivity in a high UV-B environment. This would also suggest that ecotypic differentiation may have occurred in response to increasing UV-B radiation over an elevational gradient. The range of adaptability to increased UV-B also implies changes in species and community dynamics that might be anticipated in natural plant populations if stratospheric ozone depletion continues.